Optical networks include various optical switches or nodes coupled through a network of optical fiber links. Optical network failures or faults may be caused by any number of events or reasons, including damaged or severed optical fibers, or equipment failure. Because optical fibers may be installed virtually anywhere, including underground, above ground or underwater, they are subject to damage through a variety of ways and phenomena. Optical fibers and optical equipment may be damaged or severed, for example, by lightning, fire, natural disasters, traffic accidents, digging, construction accidents, and the like.
Because optical fibers carry far greater amounts of information than conventional copper wires used to transmit electrical telecommunications signals, the loss of an optical fiber can cause far more user disruptions when compared with the loss of a copper wire. Because dozens of fibers may be routed within a single cable or conduit, a damaged or severed cable or conduit will potentially result in far greater user disruptions and outages than the loss of a cable or conduit of copper wires.
To reduce the negative effects of optical network failures, optical network topologies are provided in arrangements and configurations, such as mesh or ring topologies, so that telecommunications traffic may traverse the optical network using multiple optical links. This allows such optical networks to be reconfigured to route around network failure point. An optical network may include both working links or paths and spare links or paths that may be used to assist with optical network restoration. The optical switches of the network may be programmed to configure their ingress and egress ports based on a switch state table. During optical network restoration, these switch state tables must be modified, changed or implemented to route telecommunications traffic around the failure and to minimize the loss of telecommunications traffic. Because of the large amount of data or bandwidth an optical network carries, the amount of time it takes to identify the location of an optical network failure, and the time it takes then to reconfigure the optical network, significant amounts of telecommunications traffic are often lost. Further, the reconfiguration of an optical network may result in the loss of other telecommunications traffic if not done efficiently or optimally. The capability to detect failures in an optical network is generally considered more difficult than detecting a failure in a telecommunications network that operates in the electrical domain. This failure detection difficulty normally results in increased time to locate a network failure, which further complicates the capability to efficiently restore a failure in an optical network.
The capability to quickly recognize a fault or failure and to efficiently and quickly restore normal traffic flow is crucial to the overall performance and reliability of the optical network, which is critical to an optical network operator, such as a telecommunications carrier or business enterprise, and its users. Prolonged outages result in decreased network revenues, business losses by commercial users, and an overall reduction in user confidence and satisfaction. For example, the loss of a single optical link, such as an optical link carrying a Wavelength Division Multiplexed (“WDM”) signal, may result in the loss of hundreds of thousands of phone calls and computer data transmissions.
Prior restoration techniques and methodologies were generally designed for restoring telecommunications networks operating in the electrical domain as opposed to the optical domain, which presents additional challenges. Unfortunately, each of these techniques suffer from significant problems and disadvantages.
One technique involves the use of a central control and central database to model the network, to monitor network operations, and communicate instructions to each node or OCCS in response to a failure. The central control and dispatch model suffers from a serious speed disadvantage because of the time it takes to detect failures, communicate failures to the central control, to calculate an acceptable solution, and then to communicate back to each node a desired switch configuration. Often, the central control approach is inaccurate because the most recent model of the network is not used. This approach is not acceptable in most optical networks, especially WDM optical networks.
An improvement, in some respects, on the central control technique is the use of central control with predetermined switch configurations for various potential optical network failures. This provides faster restoration because of the elimination of the need to calculate an acceptable solution each time a failure occurs. This solution still may suffer significant disadvantages, such as excessive time to contact each such OCCS with the needed provision information and the results may be based on bad or inaccurate network model information.
Another technique includes the use of a Distributed Restoration Algorithm(“DRA”). When DRA is used, it is assumed that the various cross connect switches can communicate messages to each other in a peer-to-peer fashion. This may be achieved through the use of excess network capacity, through a signaling network or the use of a separate wide area network. These messages may include flooding messages where, when a connection fails or is lost, a sender node or cross connect switch sends out a message that will eventually be passed around the network until the message is received at the other node of the lost link. In this manner, the two affected nodes or switches can communicate and a second path can be setup to work around the failed link. Unfortunately, for optical networks, this technique of messaging before the link is restored takes too long. Response times as short as 100 to 200 milliseconds may result in dropped traffic. Another disadvantage is that the various nodes may get into a loop condition where the nodes are contending against one another for network resources.